This invention relates to broadband radio frequency communication systems and methods and more particularly to a system and method for optimizing capacity in multi-user time division duplex communication systems through minimizing guard times utilized in the duplexed signals.
In the past, information communication between processor-based systems, such as local area networks (LAN) and other general purpose computers, separated by significant physical distances has been an obstacle to integration of such systems. The choices available to bridge the physical gap between such systems have not only been limited, but have required undesirable tradeoffs in cost, performance, and reliability.
One group of historically available communication choices includes such solutions as the utilization of a standard public switch telephone network (PSTN) or multiplexing signals over an existing physical link to bridge the gap and provide information communication between the systems. Although such solutions are typically inexpensive to implement, they include numerous undesirable traits. Specifically, since these existing links are typically not designed for high speed data communication, they lack the bandwidth through which to communicate large amounts of data rapidly. As in-building LAN speeds increase to 100 Mbps, the local PSTN voice grade circuits even more markedly represent a choke point for broadband metropolitan area access and therefore are becoming a less and less desirable alternative. Furthermore, such connections lack the fault tolerance or reliability found in systems designed for reliable transmission of important processor-based system information.
Another historically available group of communication choices is found at the opposite end of the price spectrum than those mentioned above. This group includes such solutions as the utilization of a fiber optic ring or point to point microwave communication. These solutions are typically cost prohibitive for all but the larger users. The point to point systems require a dedicated system at each end of the communication link which lacks the ability to spread the cost of such systems over a plurality of users. Even if these systems were modifiable to be point-to-multipoint, to realize the economy of multiple system use of some system elements, the present point-to-point microwave systems would not provide broadband data services but rather traditional bearer services such as T1 and DS3. Furthermore, these systems typically provide a proprietary interface and therefore do not lend themselves to simple interfacing with a variety of general purpose processor-based systems.
Although a fiber optic ring provides economy if utilized by a plurality of systems, it must be physically coupled to such systems. As the cost of purchasing, placing, and maintaining such a ring is great, even the economy of multi-system utilization generally does not overcome the prohibitive cost of implementation.
A need therefore exists in the art of information communication for a communication system providing cost effective bridging of large physical distances between processor-based systems.
A further need exists in the art for a communication system providing high speed broadband information communication between processor-based systems.
A still further need exists in the art for a communication system and method of operation which efficiently utilizes the available spectrum in order to provide optimized information throughput.
A still further need exists in the art for a fault tolerant communication system providing reliable bridging of physical gaps between processor-based systems.
Additionally, a need exists in the art for a broadband communication system providing simple connectivity to a variety of processor-based systems and communication protocols, including general purpose computer systems and their standard communication protocols.
These and other objects, needs and desires are achieved by a system and method of communication in which a communication array (referred to herein as a hub), is centrally located to provide communication links to a plurality of physically separated subscriber processor-based systems, or other sources of communication such as voice communication, using a communication device (referred to herein as a node, which together with the subscriber processor-based system is referred to herein as a remote system or subscriber system) of the present invention to efficiently utilize available communication spectrum. Preferably, this central array may be physically coupled to an information communication backbone providing communication between air linked systems and physically linked systems. Furthermore, multiple ones of such system may be utilized to bridge large physical separation of systems by the intercommunication of multiple central arrays. Moreover, pervasive surface coverage may be provided by arranging a plurality of such communication arrays to provide a cellular like overlay pattern.
In a preferred embodiment, the communication spectrum utilized by the communication system is frequency division multiplexed (FDM) to provide multiple channels or carriers for simultaneous information communication to a plurality of subscribers. Moreover, a preferred embodiment subscriber system is adapted to be dynamically controllable to select between ones of the FDM carriers utilized by the communication system.
Preferably a carrier frequency in the millimeter wavelength spectrum, such as 10 to 60 GHz, is used by the present invention. Such carrier frequencies are desirable in order to provide a communication bandwidth sufficient for the transmission of at least 30 Mbps through each defined FDM channel of approximately 10 MHz. However, it shall be appreciated that the concepts of the present invention are applicable to portions of the spectrum other than millimeter wavelengths. For example, the present invention is particularly well suited for use in lower frequency bands, such as those in the 300 MHz to 3 GHz range, where radiation of signals are not as confined to line-of-sight as those of the millimeter wavelength spectrum.
Time division multiplexing (TDM) is preferably utilized to provide multiple, seemingly simultaneous, communications on a single carrier channel. Here ones of the FDM channels are broken down into a predetermined number of discrete time slices (burst periods) which form a frame. Each burst period may be utilized by a different subscriber so as to result in information communication contained in a single frame, having a number of TDM bursts, being directed to/from a number of subscribers over a single FDM channel.
Moreover, full duplexing may be synthesized on a single carrier channel by time division duplexing (TDD) through the use of burst periods like those used in TDM. Through TDD, Tx and Rx frames, each frame having one or more burst periods, are defined to provide communication in a particular direction at a predefined time. According to a most preferred embodiment, TDD of the present invention is adaptive (ATDD) to provide for dynamic sizing of the Tx and Rx frames. For example, allocation of burst periods to either a Tx frame or Rx frame may be based on the instantaneous traffic demands of the subscriber systems.
Preferably timing adjustment techniques are employed with respect to burst periods of a multi-user TDD frame. For example, where one subscriber system using data of a TDD frame is disposed more near a hub site and another subscriber system using data of the TDD frame is disposed more remote from the hub site, a preferred embodiment of the present invention utilizes timing advance techniques with respect to burst periods of the hub Rx frames in order to properly time align reverse link transmission at the hub site.
However, the use of timing adjustment techniques, such as the above mentioned timing advance, creates a potential for burst period overlap in multi-user systems. For example, a subscriber system operating in the first burst period of the Rx frame of a TDD frame may be advanced to the point that it begins transmission during the last burst period of the Tx frame of the TDD frame.
The above described problem with burst period overlap may be addressed by providing a sufficiently long guard time between the Tx and Rx frames of a TDD frame. However, such a solution if not carefully implemented leaves the available spectrum idle for periods of time in which communications may take place, thus decreasing the throughput theoretically possible with the available spectrum. Accordingly, the preferred embodiment of the present invention includes timing adjustment techniques adapted to efficiently utilize the available spectrum in order to provide optimized information throughput.
The preferred embodiment of the present invention enables the guard time between the forward and reverse links of a TDD carrier to be adjusted for optimization of communications. Accordingly, depending upon instantaneous traffic load associated with the subscribers utilizing a particular TDD carrier, guard times may be adjusted to provide a desired level of throughput. According to a most preferred embodiment, as traffic loads demand, the order of assignment of burst periods in either or both of the Tx and Rx frames of a TDD carrier with respect to the subscriber systems operable thereon are adjusted to accommodate desired guard times. For example, the order of subscriber systems on the reverse and/or forward links may be arranged to correspond to their distance from the hub in order to allow guard times to be reduced to a minimum. Preferably, determination of the guard time required for any such instantaneous traffic demands is based upon the smallest propagation delay associated with a subscriber system of the multiple users of the TDD carrier, i.e., the subscriber system disposed closest to the hub.
In the preferred embodiment, the communication system may utilize an algorithm, perhaps at initialization and/or continuously or periodically during use, to poll subscriber's systems or otherwise monitor communication attributes with respect to operation of hubs and nodes, and preferably various elements thereof, of the communication system. This communication attribute information may be utilized to determine the optimum assignment of resources, including antenna elements, TDM burst periods, FDM frequency assignments, and/or TDD Tx and Rx time assignments for each such system. For example, propagation delay information with respect to the nodes operating with a hub derived by the above algorithm during an initialization or acquisition process may be used to determine timing advance information and/or the distance from the hub site and the subscriber systems for use in TDD Tx and/or Rx time assignments.
A technical advantage of the present invention is provided by the optimization of communications based on the communication attributes of the system.
A further technical advantage of the present invention is provided by the determination of a minimum guard time between the forward and reverse links of a TDD carrier to optimize use of the available spectrum.
A still further technical advantage of the present invention is that variable guard times in a TDD carrier are supported to allow optimization based on the deployment of subscriber systems and/or the traffic loading of the system.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.